Regulatory T cells involved in the antibody response to Type III pnuemococcal polysaccharide (SSS-III) can be activated by the infusion of B cells primed by prior exposure to antigen (SSS-III). If such immune B cells are incubated before transfer-with appropriate amounts of F(ab')2 fragments from anti-IgM and IgG (heavy and light chain specific) antibody, or anti-IgM (chain specific) antibody, the transferred B cells fail to act as inducers for the activation of suppreessor T cells. By contrast, the treatment of immune B cells with F(ab')2 of anti-IgD (delta chain specific) is without effect. Since both antibody treated and untreated B cells localized to the spleen in the same manner after cell transfer, these findings indicate that B cell surface antibody of the IgM class plays an important role in the activation of suppressor T cells involved in the antibody response to SSS-III.